As is well-known, inkjet printers operate by ejecting droplets of ink onto a web or sheet medium. Such printers have printheads that are non-contact heads with ink being transferred during the printing process as minute “flying” ink droplets over a short distance of the order of ½ to 1 millimetre. Modern inkjet printers are generally of the continuous type or the drop-on-demand type. In the continuous type, ink is pumped along conduits from ink reservoirs to nozzles. The ink is subjected to vibration to break the ink stream into droplets, with the droplets being charged so that they can be controllably deflected in an applied electric field. In a thermal drop-on-demand type, a small volume of ink is subjected to rapid heating to form a vapour bubble which expels a corresponding droplet of ink. In piezoelectric drop-on-demand printers, a voltage is applied to change the shape of a piezoelectric material and so generate a pressure pulse in the ink and force a droplet from the nozzle.
Most inkjet printers are designed with scanning printheads. Because of the cost of manufacture, such printheads generally have a small number of nozzles. To print even a small page, the head is moved over the medium and ink droplets are ejected at the appropriate moment to construct the portion of the image being created. As only one band of an image is created in a single scan, the process is quite slow. For industrial grade inkjet printers where printing speeds of the order of 60 pages per minute are sought, inkjet printers have been developed which extend across the full width of sheet media to be printed. Of particular but not exclusive interest in the context of the present invention are thermal drop-on-demand inkjet printheads commercially available under the MEMJET registered trade mark. Such printheads use thermal energy to produce a vapor bubble in ink occupying a channel so as to expel an ink droplet from a nozzle at an exposed end of the channel. The printhead is manufactured as an integrated circuit device to include heating resistors located adjacent to the ink ejection nozzles, the resistors being individually energized by electrical heating pulses in response to an input print signal. For each ink colour or type, a separate ink supply circuit is used having an ink supply container and a peristaltic pump for pumping ink from the container to the printhead. For each ink colour/type, the printhead has an ink inlet port, an ink outlet and a main channel. Ink is drawn from the main channel into branch channels by capillary action to replace ink that is ejected in the course of printing. Printing is enabled by “firing” selected nozzles at the printhead active face. Other than when firing, ink in a nozzle chamber is prevented from escaping from the nozzle and flooding the nozzle plate by maintaining a negative hydrostatic pressure at the printhead. The Memjet printheads have a high nozzle density of the order of 1600 dots per inch (dpi). A series of such integrated circuit devices may be combined to provide a page wide printhead typically having five colour channels. Typically, the preferred Memjet integrated circuit printhead has of the order of 70,000 nozzles. At a paper speed of 12 inches (305 mm) per second, the printhead produces 1600×800 dpi quality, while at a speed of 6 inches (152 mm) per second, the printhead produces 1600×1600 dpi output for high-quality graphics (1-2 picolitres). Ink drop placement is very accurate with ink drops being of the order of 14 microns in diameter. Typically a Memjet IC chip contains 5 ink channels with two rows of nozzles per channel. Preferred Memjet devices have nozzles which are coated with a layer of silicon nitride to provide a smooth, flat surface resisting debris adhesion and so providing for ease of maintenance.
In order to keep an inkjet printhead capable of printing high quality images, certain maintenance procedures are performed during a printing process. Among such procedures is printhead capping which consists of placing a cap over the printhead nozzles when a printing operation is temporarily suspended to ensure that ink at the printhead nozzles does not dry out and cause partial or full blocking of an inkjet nozzle. Another common procedure is printhead cleaning in which ink is ejected though the printhead nozzles to flood the printhead face which is then washed in the ink. In addition, maintenance elements may include a spittoon to receive excess ink that may inadvertently flood the printhead face or may have been deliberately applied to the printhead face in the course of the cleaning process. Conventionally, the maintenance elements are mounted as an assembly, the assembly having an associated drive mechanism to bring appropriate maintenance elements to the print face when required and an ink drain means for draining excess or cleaning ink from the printhead face. Accommodation must be made for such an inkjet maintenance assembly which takes into account the position and operation of the inkjet print engine (of which the inkjet printhead is a primary part) and the inkjet printer sheet media transport mechanism.
A known arrangement of printhead engine and maintenance assembly that is particularly adapted for cut sheets is shown in FIG. 1. The printer has a transport mechanism in which cut sheets are moved through the printer using consecutive nips. At each pair of nips, a first upstream nip grips the sheet and pushes it downstream. Before the sheet has fully left the first nip, a leading edge of the sheet is gripped by a downstream nip which draws it into the downstream nip and then drives it further downstream. In one known arrangement, a plane containing the transport path for the cut sheets extends between an overlying print engine and an underlying maintenance assembly. The print engine and the maintenance assembly are located between consecutive transport mechanism nips and face each other across the transport path. To undertake a maintenance procedure, the printing process is halted at a juncture when no cut sheet medium is occupying that part of the transport path between the two consecutive nips. The particular maintenance element, such as a capper or cleaner is moved up into engagement with the printhead face and the corresponding maintenance procedure is performed.
An alternative transport equipment for transporting cut sheets to and from an inkjet print station disclosed in U.S. patent application Ser. No. 13/368,280 (Multiple printhead printing apparatus and method of operation) filed Feb. 7, 2012, the contents of which are hereby incorporated by reference in their entirety and made part of the present United States patent application for all purposes. The aforesaid application describes a printing apparatus having a series of inkjet printheads spaced from one another in a transport direction. A continuous belt driven around a roller system is used to feed sheet media successively to the printheads so that a partial image printed by one printhead is overprinted at a subsequent printhead with registration of the partial images. A sheet medium is caused to become electrostatically tacked to the belt by passing the sheet past a charging device. Movement of the belt is tracked by a tracking sub-system and operation of the printheads is coordinated with the tracked belt movement to achieve precise registration of the partial images. The nature of this transport system means that every part of the continuous belt tracks under the printheads during the printing process. Consequently, it is not possible to provide access to maintenance elements located underneath the printhead because access is blocked by the conveyor belt.